Building an autonomous drone swarm capable of nuclear-weapon-tier kill count is far easier than building a nuclear weapon. We already know how dangerous military drones are, and those are still mostly bottlenecked by relying on human-powered target identification. Building drones to indiscriminately kill unarmored civilians is an easier task.
We are literally seeing the large scale use of robotic weapons platforms AT SCALE.
Machine guns create a storm of ballistic objects, and have been doing so for over 100 years. Large scale murder isn't new. We just have systems that have cameras that make it easy to put the footage online now.
This is just as true for guns as drones. Both need fast reactions and fine control. A human cannot control a swarm of drones any better than they can control a swarm of guns, but you can add autonomy (sensors, actuators, compute) to either to get some of those capabilities back.
Conventional munitions will always be more economical. It's hard to beat a few grams of steel in price per kill. Drones are only relatively cheap when compared to rifles, but they're actually replacing bullets/bombs. Great for high value targets, less great for indiscriminate killing.